Various types of current meter have been used in the past to gauge the volumetric flow race of rivers. Typically the river velocity is measured by a propeller or other rotating device, suspended with a weight from a cable. This is done at intervals across a river. The velocity is calculated from the rate of rotation of the device and the river depth is measured by noting the length of cable wound out. Flow rate is the product of velocity and cross-sectional area, which is calculated from the measured depth and the horizontal distance between measurements.
This technique has serious disadvantages. Firstly, in high flows the propeller type meter is carried downstream by its own drag, drag on the weight and cable and drag from debris caught on the meter assembly. Thus the meter does not hang vertically below the gauging point and it is usually necessary to apply a vertical angle correction to estimate the true depth of flow.
Secondly, the drag increases with the square of the river velocity and becomes so great that it is not practical to measure river velocities above flow rates of about 18 ft/sec with the traditional equipment.
Thirdly, because propeller type meters have inertia, in a fluctuating velocity field the measured velocity results from the damped time average of the disturbing forces acting on the rotating device. Because velocity fluctuations are related to the square root of these forces, where the disturbing forces are not evenly distributed about a mean value, as occurs in the wake of any protrusion into the flow, the time average of the forces converted to a velocity, is not the same as the average of individual velocity fluctuations. Thus the propeller type device gives a biased estimate of average velocity. For example, suppose that disturbing forces of 9 units, -1 units, 0 units and -4 units occur over successive short time intervals and these forces arise from velocity fluctuations of 3 ft/sec., -1 ft/sec., 0 ft/sec. and -2 ft/sec. The true mean of the fluctuating velocity is the sum (3-1+0-2) divided by four which equals zero feet per second. Conventional propeller type meters average the disturbing forces (9-1+0-4 divided by four), which corresponds to an incorrect mean of one foot per second for the fluctuating velocity.
Fourthly, the mean velocity at a fixed depth below the surface generally varies depending on the depth at which a measurement is made in a river. Conventional meters are used at a fixed depth or depths and standard assumptions are applied to relate the fixed depth velocity to the true depth--averaged mean velocity. Whilst these assumptions are applicable to a "standard" river situation, many rivers are non-standard or become non-standard at certain flow rates. The only way to determine whether a measurement at 0.6 of the total depth is representative of the mean velocity is to take readings at other depths, say 0.2 and 0.8 of the total depth. The only way to be sure that these three readings are truly representative is to take even more fixed depth measurements. This is a time consuming and therefore costly process
Finally, the inherent dangers of flood gauging and weather conditions usually mean that flow calculations are delayed until an operator reaches shelter. If errors are found at this point in is often too late to repeat a gauging measurement.